This proposal seeks to advance knowledge regarding adolescent life course determinants of HIV risk behavior with explicit attention to the neighborhood context of risk. While a range of studies has investigated the role of structural context (e.g., neighborhood poverty) on the prevalence of risk behavior, few have attempted to unpack the mechanisms that link these factors with rates of risky sexual activity. We address the following questions: (1) Are structural and spatial features of urban neighborhoods (concentrated disadvantage, concentrated affluence, residential stability, & ethnic heterogeneity) associated with adolescent sexual risk behavior? (2) Do neighborhood social processes (collective efficacy, physical and social disorder, & normative orientations) mediate or moderate associations between neighborhood structural effects and sexual risk behavior? (3) Do community institutional resources (neighborhood organizations/services & schools) mediate or moderate associations among neighborhood structural effects and sexual risk behavior? And (4) Do neighborhood social networks and family processes (conflict/aggression and parental warmth, harshness, and engagement) mediate or moderate neighborhood structural effects on sexual risk behavior? We also consider whether the effect of hypothesized mechanisms of neighborhood structural effects on sexual risk behavior are moderated by individual developmental vulnerabilities (poor emotional health and low cognitive functioning) and proximal risk factors (exposure to violence, deviant peers, & risk tolerant attitudes) as well as potential interactions between neighborhood social processes and other contextual influences. Data from the Project on Human Development in Chicago Neighborhoods are used to examine neighborhood effects on sexual risk behavior. This study is multilevel, with households randomly selected from 80 Chicago neighborhoods stratified by race/ethnicity and SES. For this proposal, 3 waves of data will be used for 3 cohorts of youth first seen at ages 9, 12, and 15 and every 2 years thereafter (N = 2344). Intensive interviews with adolescents and parents were conducted at each wave. Neighborhood characteristics were assessed before wave 1 and at wave 3. Neighborhood data are drawn from: (1) large-scale survey of resident's assessments of neighborhoods; (2) observational study of neighborhoods; (3) Census data; and (4) administrative data from local agencies.